Chromium base alloy possessing high strength at elevated temperatures



s No Drawing.

United States Patent F 3,137,572 CHROMIUM BASE ALLOY POSSESSING IHGH STRENGTH AT ELEVATED TEMFERATURES Lewis R. Aronin, Lexington, and Arthur L. Gear-y, Ar-

lington, Mass, assignors, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Original application June 30, 1960, Ser- No. 40,123, now Patent No. 3,085,007, dated Apr. 9, 1963. Divided and this application Feb. 14, 1963, Ser.

3 Ciaims.

The present invention relates in general to chromium rich alloys and is specifically directed to alloys having superior strength at high temperatures combined with adequate hot workability.

This application is a division of application Serial Number 40,123 filed on June 30, 1960, now US. Patent Number 3,085,007, issued April 9, 1963.

Chromium, because of its high melting point and oxidation resistance, offers an attractive basis for high temperature alloys. Strengthening achieved by selected alloying elements should result in an alloy having useful strength as a structural material at temperatures well in excess of 1000 C. (1830 F.). While other metals such as molybdenum, tantalum and tungsten have higher melting points than chromium, the latter is superior to these in oxidation resistance.

In the search for strong, high temperature chromium alloys, the most satisfactory alloy has been reported as CrFeMo in the range 60 w/o chromium, 15 to 20 w/o iron and 15-25 w/o molybdenum. (Parke and Bens, ASTM Symposium on Materials for Gas Turbines, 1946.)

Sully and Brandes, The Properties of Cast Chromium Alloys at Elevated Temperatures, Journal of the Institute of Metals, 81, 573 (1953), also investigated the strengths of cast chromium base alloys. Results of compression and creep tests at 900 C. (1650 F.) indicated that a Cr10 w/o Fe10 w/o W alloy containing either 5 or W/o Ta was the most resistant to deformation when stressed at 10,000 p.s.i. Erratic results, however, were obtained in tensile stress-rupture tests, this behavior being attributed to defects in the cast structure which obscured the intrinsic creep resistance of the alloys. Chromium base alloys containing 1 w/o Ti and up to 5 w/o W prepared by are melting mixture of hydrogentreated electrolytic chromium and high purity titanium and tungsten powders have been successfully forged, swaged and rolled (Henderson, Quass and Wain, The Fabrication of Chromium and Some Dilute Chromium- Base Alloys, Journal of the Institute of Metals, 83, 126 (1954). Other workers report that Cr-5 w/o Mo and Cr10 w/o W alloys fabricated by extrusion of arc-cast ingots, possess attractive creep and stress-rupture properties at 950 C. (1740 F.). (Wilms and Rea, Preliminary Investigations on the Properties of Chromium and Chromium Alloys at Elevated Temperatures, Journal of the Institute of Metals, 87, 77 (1958).)

It is the object of this invention to provide chromium alloys which have superior strength at high temperatures, are hot workable and extrudable.

These objects are obtained by alloying the chromium with small quantities of columbium, tantalum, and/or molybdenum. The alloys are distinguished from the' prior art in that high-temperature strength is achieved with small alloying additions. Only two of the alloys of this invention have alloy contents as high as 7 W/o, while the majority have alloy contents substantially less than 5 w/o. The alloys can be hot worked and are extnldable into a Wrought product.

FABRICATION OF ALLOYS The alloys of the subject invention are fabricated by Blended powders, com-- powder metallurgy techniques. pacted at 50,000 p.s.i. into 1-inch diameter by Z-inch high slugs, are sintered for 3 /2 hours in purified argon at 2685-2730 F. (1475-1500 C.). The sintered slugs are enclosed in type 304 stainless steel containers and evacuated to 0.01 micron while heating at temperatures up to 800 F. (430 C.).. After heating one hour at 2400 F. (1315 C.), the billets are extruded from a 2.800 liner of a 1000-t0n press through 0.800 inch dies. The alloy core so produced is approximately .25 inch in diameter.

Alternatively, the slugs may be sintered in hydrogen. Slugs sintered in hydrogen are evacuated one hour at 1830 F. (1000 C.) to reduce residual hydrogen. In these alloys high purity chromium is used.

TESTING THE ALLOYS In order to test the alloys at a temperature of 2300 F. (1260 C.) a conventional stress-rupture test machine was modified to permit use of an insert (argon) atmosphere. Specimens are placed in grips and inserted in a tubular furnace which is comprised of a Kanthal winding on a mullite tube. The latter is sealed, flushed with argon, after which the furnace is turned on. Within about 5 hours the specimen reaches the test temperature, 2300 F. (1260 C.). After soaking for half an hour at temperature, a load is applied to the specimen and the rupture life is measured to the nearest 0.1 hr. by an electric timer.

The properties of unalloyed chromium and 23 chromium-base alloys have been investigated in stress-rupture tests at 2300 F. (1260 0.). Results for unalloyed chromium and for chromium alloys are set forth below in Table I and Table H respectively:

Table I.-Results of Stress-Rupture Tests at 2300 F. (1260 C.) Unalloyed Chromium COMPACTED POWDERS EXTRUDED TO 0.18-INCH DIAMETER Elonga- Reduc- Rupture Atmosphere Stress tion tion in Life (p.s.i (percent) Area (hr.)

(percent) COMPAOTED POWDERS EXTRUDED TO 0.25-INCH DIAMETER In order to determine which alloys have a stress-rupture life of 100 hours or more at 1000 p.s.i. and at a temperature of 2300 F., Table III has been compiled by interpolation and extrapolation of log-lot plots of rupture life vs. applied stress. While such extrapolations assume that chemical and metallurgical instabilities do not alter the slope of the curve, over short distances such assumptions are valid, and the data summarized in Table III is deemed to be correct within reasonable limits.

Table III.Life at 1000 p.s.i. and Stress to Rupture iii 100 Hours at 2300 F. Unalloyed Chromium and Chromium-Base Alloys 100 Hour Alloy Composition (a/o) Life at 1000 Rupture p.s.i. (hr.) Stress (p.s.i

(Jr-2.0 Ia0.l M0 250 (1270 Cr1.0 (lb-0.1 M0 220 1260 Cr1.0 Cb i 130 1100 Cr1.0 Cb0.1 Ta 130 1100 (Jr-2.0 Ta0.1 Cb0.1 M0 122 1070 (Jr-1.0 Tit-0.1 M0 (104) (1020) Cr1.0 Clo-1.0 M0. (103) (1020) 68 830 64 800 800 (63) (800) (54) (770) 50 840 48 551 48 770 35 550 33 590 590 33 590 23 510 23 510 23 510 12 320 Unalloyed Chromium:

Compaeted-OJS-inch core (17) (600) Compacted0.18-inch core and tested in helium 17 600 Sintered0.18-inch core (33) (710) Compaeted0.25-inch core 450 Oompacted-0.25-lneh core and heat treated 10 450 Sintered0.25-i uch core (71) (910) Doubtful values enclosed in parentheses.

From Table III it Will be seen that appropriate additions of Cb, Ta and Mo have resulted in rupture lives 3 to 5 times that of unalloyed chromium. Quantitatively, the rupture lives of certain of these alloys substantially exceed the desired hour rupture life at 1000 p.s.i. and at a temperature of 2300 F. Of particular interest is the alloy Cr-1 a/o Cb-0.1 a/o Me which appears to be the strongest alloy tested except perhaps the Cr2 a/o Ta-0.l a/o Mo as to which the data is scattered so that its life at 1000 p.s.i. cannot be precisely stated.

A number of other alloys successfully meet the desired conditions. These include (in atomic percent) Cr1.0 Cb; Cr-l.0 Cb0.1 Ta; Cr2.0 Ta-Oul Cb-0.l Mo; Cr1.0 Ta0.1 Mo; and Cr1.0 Cb-l.0 Mo.

From the foregoing it is seen that new and useful alloys of chromium have been produced by the addition of small amounts of selected alloying elements whereby the strength of chromium has been substantially increased at stresses of about 1000 p.s.i. and temperatures of about 2300 F.

We claim:

1. An alloy of chromium characterized by high strength at temperatures of about 2300 F. that consists of chromium-1 atomic percent tantalum, and 0.1 atomic percent molybdenum.

2. An alloy of chromium characterized by high strength at temperatures (I about 2300 F. that consists of chromium-2.0 atomic percent tantalum, and 0.1 atomic percent molybdenum.

3. An alloy of chromium characterized by high strength at temperatures of about 2300 F. that consists of chromium2.0 atomic percent tantalum, 0.1 atomic percent columbium and 0.1 atomic percent molybdenum.

No references cited. 

1. AN ALLOY OF CHROMIUM CHARACTERIZED BY HIGH STRENGTH AT TEMPERATURES OF ABOUT 2300*F. THAT CONSISTS OF CHROMIUM-1 ATOMIC PERCENT TANTALUM, AND 0.1 ATOMIC PERCENT MOLYBDENUM.
 3. AN ALLOY OF CHROMIUM CHARACTERIZED BY HIGH STRENGTH AT TEMPERATURES OF ABOUT 2300*F. THAT CONSISTS OF CHROMIUM - 2.0 ATOMIC PERCENT LANTALUM. 0.1 ATOMIC PERCENT COLUMBIUM AND 0.1 ATOMIC PERCENT MOLYBDEUM. 